1. Field of the Invention
The present invention relates to isotopic analysis. In particular, the present invention is a method and apparatus for determining the relative amounts of isotopes of hydrogen by atomic line emission spectra.
2. Discussion of Background
Isotopic analysis of hydrogen is used throughout the nuclear industry and, in particular, in processes where two isotopes of hydrogen, deuterium and protium, or their compounds, heavy water and ordinary water, respectively, must be separated or kept separated. Such processes include heavy water refining and reprocessing, environmental testing (in which deuterium may be used as a non-radioactive tracer), and monitoring of heavy water reactor moderator for possible light water contamination. Similarly, in the processing of the third isotope of hydrogen, tritium, isotopic analysis is needed to monitor the purity of the product and to guard against contamination by either of hydrogen's lighter isotopes. In some cases, it is necessary to detect even very small amounts of one hydrogen isotope in the presence of very much larger amounts of another.
Most hydrogen isotopic analysis is currently performed using mass spectroscopy: a sample is injected into a vacuum chamber, where its atoms are ionized and accelerated in one direction, forming a beam. After moving through a combination of electric and magnetic fields, the ions in the beam are sorted out according to their mass, charge and velocity, with some of them being steered into a detector. By varying field strengths, it is possible to steer first one and then another part of the beam into the detector, and measure the relative number of ions in each part.
Mass spectroscopy is a cumbersome process, requiring equipment which typically costs several tens of thousands of dollars. The magnets are heavy and bulky, and must be set up very precisely so as to generate fields of uniform and well-known characteristics. Constant pumping is needed to maintain a high-quality vacuum (about 10.sup.-6 mm Hg) inside the chamber, all the while the sample material is continuously being injected into it, and notwithstanding the inevitable leaks and outgassing of system components. High ion-accelerating voltages are needed, creating a possible hazard for operators and maintenance people. Additionally, health risks, not yet fully understood, have been associated with strong magnetic fields.
Because the vacuum system must be pumped down thoroughly between samples and because each sample is analyzed literally "atom-by-atom", the method is slow. Further delays result from the fact that, due to the high cost, a single mass spectroscope must often be shared among several different experiments or sampling points. Hence, mass spectroscopy is not well-suited to real time applications such as advanced process control.
Mass spectroscopy is further limited in that it cannot readily separate isotopes of similar mass but different atomic number. While this is not a major problem in most hydrogen isotopic analyses, it is significant in the case of tritium since its decay product is helium 3. In some cases, charged molecules can also "masquerade" as ions of heavier isotopes: for instance, HD.sup.+ for tritium. There remains a need for a simple, effective apparatus for the isotopic analysis of hydrogen.